The technical field of this invention is the implantation of ions into targets, such as silicon semiconductors, and, in particular, to improved methods for generating a silicon heterostructure having a discrete, continuous, buried dielectric layer which is substantially free of defects.
Ion implantation techniques are particularly useful in forming a class of buried layer devices known as silicon-on-insulator (SOI) devices. In these devices, a buried insulation layer is formed beneath a thin surface silicon film. These devices have a number of potential advantages over conventional silicon devices (e.g., higher speed performance, higher temperature performance and increased radiation hardness).
In one known technique, known by the acronym SIMOX, a very thin (0.1 micron-0.3 micron) layer of monocrystalline silicon is separated from the bulk of the silicon wafer by implanting a high dose of oxygen ions (e.g., up to about 3.0.times.10.sup.18 oxygen ions/cm.sup.2) into the wafer to form a buried dielectric layer of silicon dioxide (having a typical thickness ranging from about 0.1 micron to 0.5 micron). This technique of "separation by implanted oxygen" (SIMOX), provides a heterostructure in which a buried silicon dioxide layer serves as a highly effective insulator for surface layer electronic devices.
While SIMOX technology is proving to be one of the most promising of the SOI technologies, there are a number of problems still associated with the manufacturing of SIMOX materials, as practiced in the art. One particular problem is the presence of isolated pockets of silicon within the buried dielectric layer. These silicon inclusions or "islands" tend to form near the lower interface of the buried layer with the underlying bulk silicon region and can severely affect the performance of SIMOX devices, particularly as MOS devices, by acting as overlapping floating gates. Once formed, the silicon islands are difficult to eliminate, primarily because of silicon's slow diffusion rate, relative to that of oxygen.
Implanting lower doses of oxygen ions appears to exacerbate silicon island formation, particularly as the dose approaches the critical implant dose (the minimum implant dose required to produce a continuous amorphous layer of implanted oxygen, e.g., 1.4.times.10.sup.18 ions/cm.sup.2). However, increasing the implant dose alone does not appear to be sufficient to prevent silicon island formation, although it may reduce the number formed. In addition, higher implant doses require longer implantation times, and increase the damage to the overlying silicon body.
U.S. Pat. No. 4,749,660 (Short et al., filed Nov. 26, 1986), discloses a method of manufacturing SIMOX material having "substantially homogeneous, relatively thin buried silicon dioxide layers". The method comprises implanting a subcritical dose of oxygen ions, followed by at least one "randomizing implant" (e.g., of silicon ions) and then annealing at a low temperature. The effectiveness of this method is unknown, as no experimental evidence is provided.
EPO 298,794 (Margail, J. et al, filed Jun. 13, 1988), discloses a method of manufacturing SIMOX heterostructures having sharp interfaces, using a multiple implant protocol. The method involves implanting a total dose of at least 1.5.times.10.sup.18 ions/cm.sup.2 in a series of multiple partial implants using a constant beam energy and subjecting the wafer to a high temperature annealing protocol between each partial implant. The intermediate annealing steps are thought to reduce the buildup of threading dislocations in the overlying silicon.
There exists a need for a method of manufacturing SIMOX materials having a buried insulating layer substantially free of silicon islands, and that is rapid and cost efficient to produce. It is therefore an object of this invention to provide an improved method of manufacturing SIMOX materials having these characteristics and requiring only a single implant and anneal sequence. Another object of the invention is to provide a method of manufacturing SIMOX materials requiring a lower dose of oxygen ions. Other features and objects of this invention will be apparent from the description, figures and claims which follow.
As used herein, "overlying silicon body" is understood to mean that portion of the silicon wafer (substrate) lying over the buried oxide layer, and on which the semiconductor device is to be built. "Bulk silicon region" is understood to mean that portion of the silicon wafer (substrate) lying below the buried oxide layer. "Upper interface" and "lower interface", respectively, refer to the boundaries separating the buried oxide layer from the overlying silicon body and the underlying bulk silicon region. "Silicon islands" and "silicon inclusions" are understood to mean isolated pockets of silicon within the amorphous dielectric layer. "Substantially free of silicon islands" is understood to mean a buried oxide layer sufficiently depleted of silicon islands such that the performance of the insulating layer or the overlying device is not affected.